list(TRANSFORM): Add PREDICATE selector

Add a new `PREDICATE` selector to `list(TRANSFORM)` that uses a
user-defined function or macro to decide which elements to transform.

The predicate callable receives each element value and an output
variable name.  It must set the output variable to a truthy or falsy
value in PARENT_SCOPE; only elements for which the predicate returns
true are passed to the transform action.

Adds a shared PredicateEvaluator helper class used by both TRANSFORM
PREDICATE and (in a subsequent commit) FILTER PREDICATE.

Includes parser wiring, error and success tests covering all existing
actions (TOUPPER, TOLOWER, REPLACE, STRIP, GENEX_STRIP, APPEND,
PREPEND, APPLY) combined with the PREDICATE selector.

Issue: #27761

Help/command/list.rst

@@ -316,6 +316,39 @@ Modification
 
         list(TRANSFORM <list> <ACTION> REGEX <regular_expression> ...)
 
+    ``PREDICATE``
+      Specify a user-defined callable as a predicate.
+      Only elements for which the callable returns a true value will be
+      transformed.
+
+      .. code-block:: cmake
+
+        list(TRANSFORM <list> <ACTION> PREDICATE <function> ...)
+
+      .. versionadded:: 4.4
+
+      ``<function>`` is a user-defined :command:`function` with exactly
+      two formal parameters: the input value and the name of an output
+      variable.  The callable must set the output variable to a boolean
+      value.  Standard CMake boolean evaluation is used.
+      If the callable does not set the output variable, it is an error.
+
+      Example:
+
+      .. code-block:: cmake
+
+        function(is_relative path result)
+          if(NOT IS_ABSOLUTE "${path}")
+            set(${result} TRUE PARENT_SCOPE)
+          else()
+            set(${result} FALSE PARENT_SCOPE)
+          endif()
+        endfunction()
+
+        set(search_paths /usr/include src lib /opt/lib)
+        list(TRANSFORM search_paths PREPEND "${CMAKE_CURRENT_SOURCE_DIR}/"
+             PREDICATE is_relative)
+
 
 Ordering
 ^^^^^^^^